The present invention relates generally to power modulators and in particular to a power modulator and method of operating a power modulator.
Switched-mode modulators are often used for power supply modulation in radio transmitters as a means to improve the efficiency of RF power amplifiers. In either the envelope tracking (ET) technique, or envelope elimination and restoration method (EER), an efficient switching power supply is used to vary the supply voltage of a power amplifier according to a time varying envelope signal. The power amplifier is operated at or near saturation, where its efficiency is optimal.
It is well known that the frequency of the switching power modulator must generally be at least 10 times the modulation bandwidth of the amplified signal so to accurately reproduce the envelope signal and ease filtering requirements. Where large signal bandwidths are used, the required switching rate may exceed that practically attainable, or may be high enough that significant switching losses are incurred.
In order to address this issue, prior art solutions have attempted to control the supply voltage using a combination of a switch-mode converter stage for gross power level control and a subsequent linear regulator stage for precise power envelope control. Such a prior-art solution is shown in FIG. 1 and described in detail in the International Application WO 00/48306 HIGH-EFFICIENCY AMPLIFIER AND BURST CONTROL. As shown in FIG. 1, amplifier 101 is preceded by linear regulator 103 and switching modulator (or switch-mode converter) 105. Although the above solution does allow for more efficient operation of amplifier 101, a problem exists in that for wideband, high-speed communications systems (e.g., those using orthogonal frequency division multiplexing (OFDM) modulation), the envelope bandwidth can be greater than the maximum useful switching frequencies of switch-mode tracking supplies. Therefore, a need exists for a power modulator that can efficiently operate within a wideband, high-bandwidth communication systems.